Microporous membranes useful as filter materials are frequently made from materials which are thermoplastic polymers, an example of which is PVDF. Membranes made from PVDF are generally chemically inert and, as such, are useful for filtration of a wide variety of fluids. However, membranes made from PVDF are not inherently wettable by water. The natural hydrophobicity of PVDF membranes limits their usefulness in the filtration of aqueous solutions. In practice, this limitation is overcome by treating the PVDF membrane by a process which modifies its exposed surfaces to make it hydrophilic.
Many such processes have been described in the art. In U.S. Pat. No. 4,774,132, a PVDF membrane is first treated with strong alkali following which a polymer of acrylic acid is grafted onto its surface. In U.S. Pat. No. 4,341,615, a crosslinked polymer formed from acrylic acid, a multiply unsaturated crosslinking agent and a free-radical polymerization initiator is formed in situ in the porous structure, thereby imparting hydrophilicity to the membrane. Other processes for rendering a PVDF membrane hydrophilic are also known to those skilled in the art of membrane and filtration technology.
Different wetting properties are required for different applications of membranes. Some applications require wettability by liquids of extremely high surface tensions, while others only require wettability by water. Such different degrees of wettability can be imparted to hydrophobic PVDF membranes by selection of an appropriate process such as one of those cited above. In order to be useful as a filtration material, however, the wetting properties of the membrane should be as uniform as possible and the membrane should also be as mechanically sound as possible. While the processes known to make PVDF membranes hydrophilic are generally successful, the product is not always uniform with respect to its wetting and mechanical properties. This is generally a reflection of the non-uniform nature of the starting material, i.e., the hydrophobic PVDF membrane.